1. Field of the Invention
The present invention concerns a catalyst used as an ozone decomposing catalyst for catalytically decomposing to eliminate ozone contained in a gas and/or an ozone deodorizing catalyst for catalytically oxidizing to decompose and eliminate offensive odor ingredients contained in a gas under the presence of ozone, as well as a method of preparing such a catalyst.
2. Description of the Prior Art
Since ozone has a strong oxidizing performance and converts itself into harmless oxygen when it is decomposed, it has generally been utilized in various fields with aims for deodorizing, sterilization, bleaching, reduction of COD in effluents or the like. However, since part of ozone utilized in the above-mentioned application use is released as it is, being unreacted, into atmospheric air, it may possibly cause secondary public pollution such as a photochemical smogs. Further, if an aircraft flies through the stratosphere since air containing ozone is drawn into the interior of the aircraft, it may resultant a danger of giving undesired effects on passengers and crews. Furthermore, various kinds of high voltage generation devices, for example, dry process copying machines have been used widely in recent years and ozone released from such devices, though little, may contaminate the inside of rooms and brings about an undesired situation not negligible from a circumstantial sanitary point of view.
Ozone not only gives uncomfortable feeling by its odor but also shows a strong toxicity to a human body, which attacks a respiratory organ at a concentration above a certain level and it is extremely harmful when inhaled for a long period of time even if it is in a trace amount. In view of the above, it has been demanded for the establishment of a technique for decomposing to eliminate spent ozone released from various generation sources.
As existent methods of disposing spent ozone, there have been known, for example, (I) a treating method with activated carbon, (II) a treating method by liquid chemical cleaning, (III) a treating method by thermal decomposition and (IV) a treating method with an ozone decomposing catalyst. Among them, treatment by using the ozone decomposing catalyst is considered most advantageous for the ozone decomposition since it is free from ignition or explosion danger, requires no waste water treatment and can decompose to eliminate ozone at a reduced cost.
As the ozone decomposing catalyst, manganese oxides have been well-known so far and various methods have been developed for the preparation thereof. For instance, there has been known a method of forming manganese oxides, for example, by immersing an inorganic support such as metal, asbesto or ceramic or activated carbon with a solution of manganese compound such as manganese nitrate and, subsequently, applying a heat treatment (Japanese Patent Laid Open Publication Sho 63-197524) or a method of coating manganese oxide such as an activated manganese dioxide or electrolysis manganese dioxide on a support material (Japanese Patent Laid Open Publication Sho 61-101230). However, it has been necessary to use such existent ozone decomposing catalysts by elevating temperature by heating or removing moisture since their activity is low at low temperature or the activity is reduced remarkably upon use in a highly humid condition. Further, since the performance is low only with manganese oxide, it has been used at present while enhancing the catalytic activity by adding a transition metal such as Co, Cu, Ni or Ag or a platinum group such as Pt, Pd or Rh as a cocatalyst. In particular, in recent years in which circumstantial problems have attracted an attention, it has been demanded for a catalyst having a higher activity than in the existent products also in the ozone decomposing catalyst.
On the other hand, public pollution caused by offensive odor has been closed-up as a social problem and a deodorizing technique for removing ingredients attributable to the offensive odors have been investigated from various aspects. As deodorizing methods practiced so far, there are, for example, (I) water washing, (II) chemical cleaning, (III) adsorption, (IV) direct combustion, (V) catalytic combustion and (VI) ozone oxidization, but they have merits and demerits respectively. Among them, the ozone oxidization method of disposing the offensive odor ingredients by making use of the powerful oxidizing effect of ozone is advantageous since it can be practiced at a relatively low temperature about at a room temperature and at a reduced running cost as compared with the various methods described above. However, the method involves a drawback of requiring a long and voluminous reaction zone since the rate of reaction between ozone and offensive odor ingredient in a gas phase is low. Further, since unreacted ozone is released in atmosphere, it also suffers from a drawback of causing secondary public pollution such as a photochemical smog.
As a means for overcoming the foregoing drawbacks in the ozone oxidization, there has been proposed a method of utilizing a catalyst. In this method, offensive odor ingredients are disposed through decomposition by means of a catalyst under the presence of ozone (hereinafter referred to as ozone deodorizing catalyst). According to this method, ozone reacts with the offensive odor ingredients rapidly requiring no large reaction space and the deodorizing efficiency is improved outstandingly, as well as ozone is decomposed completely to eliminate the worry that unreacted ozone is released into the atmospheric air.
For the ozone deodorizing catalyst, various techniques have been developed so far, but their effects can not be attained to a full extent depending on the working conditions. That is, there is a drawback that the activity of the catalyst is reduced in an early stage if the catalyst is used continuously for a long period of time or used under a high humidity condition. In view of the above, a catalyst having a higher activity than the existent catalyst has been demanded.